Polyester containers as represented by polyethylene terephthalate (PET) containers have now been widely used for a variety of applications owing to their excellent properties such as transparency, heat resistance, gas-barrier property, etc.
In recent years, on the other hand, it has been strongly demanded to re-use the resources. Concerning the above polyester containers, too, attempts have been made to recover the used containers and re-use them as a recycled resin for a variety of applications. Here, as for the contents to be contained in the packaging containers, those contents that are subject to be degenerated by light, such as some kinds of beverages, medicines and cosmetics are provided being contained in opaque containers formed by using a resin composition which is obtained by blending a resin with a coloring agent such as pigment, etc. From the standpoint of re-using the resources, however, it is not desired to add a coloring agent (since the coloring agent makes it difficult to maintain the recycled resin transparent), and it has been urged to use transparent containers. Therefore, even those opaque containers adapted to containing photo-degenerative contents must be improved so as to be re-used.
In order to impart light-shielding performance (opaque property) without adding coloring agent, it can be contrived to provide a foamed container by making the bubbles present in the container wall, and various foamed plastic containers have now been proposed.
For instance, a patent document 1 is disclosing a foamed molded article in which an average size of foamed cells has a gradient, and an average size of the foamed cells present on the outer surface side is smaller than an average size of the foamed cells present in the inside thereof.
A patent document 2 discloses a plastic container having bubbles wherein when the size of the bubbles as viewed from the front surface of the container is defined by an average bubble size of long diameters and short diameters of bubbles (foamed cells), not less than 80% of bubbles have an average bubble size of not larger than 200 micrometers, and bubbles as viewed from the front surface of the container occupy an area ratio of not less than 70%.
A patent document 3 proposes a method of producing a container-like foamed molded article by injecting and filling a synthetic resin containing a foaming agent in a mold cavity which is maintained in a pressurized state with a gas to form a tubular parison (preform) having a smooth surface which is not almost foamed though the foaming agent is contained, cooling at least a portion of the surface layer of the preform, transferring the preform in a state where the inner core thereof has not been cooled yet into a large mold cavity so as to be molded and foamed by the compressed gas and/or vacuum.
A patent document 4 discloses a method of producing an integrally molded body of a partly foamed resin by utilizing the microcellular technology proposed by the present applicant. According to this production method, an integrally molded body (e.g., preform for forming a container) of a thermoplastic resin is formed, and is caused to imbibe a gas. Thereafter, the obtained gas-imbibing molded body is selectively heated so as to be selectively foamed by the formation of bubbles due to the imbibed gas, to thereby produce the integrally molded body of the partly foamed resin having foamed regions and non-foamed regions.
Patent document 1: JP-A-2005-246822
Patent document 2: JP-A-2003-26137
Patent document 3: JP-B-62-18335
Patent document 4: JP-A-2007-320082